The present invention relates in general to stereoscopic photography and more particularly concerns novel methods and means for stereoscopically photographing a scene and viewing that scene with remarkable realism, the stereoscopic field that is viewed being embraced by an exceptionally large viewing angle. An observer may adjust the interpupillary distance to suit him easily and quickly and also adjust focus for each eye separately. The illumination system uniformly backlights the slides being viewed with great efficiency from a single flashlight bulb.
Stereoscopic photography and viewing is well known. In a typical system the stereoscopic slides are framed in a common frame a fixed distance apart. While such stereoscopic systems provide pictures that are more pleasing than ordinary two dimensional slides, they are less pleasing than desired. The observer sees a scene that is three dimensional and rectangularly framed by a visible border within a relatively small solid angle, much as one sees a scene on a stage in a theatre instead of the circularly framed wide angle scene that one observes in real life. Such borders may comprise slide aperture edges or similar limiting means. Many such prior art instruments are described in Judge, "Stereoscopic Photography", 3d Edn. Rev. (Chapman & Hall Ltd., London, 1950). For a general analysis see Hardy and Perrin, PRINCIPLES OF OPTICS (McGraw-Hill, 1932), Chapter XXV, STEREOSCOPY, particularly Section 191 on The Theory of Stereoscopy.
Although they include means for accommodating different interpupillary distances, most prior art stereoscopes keep the left and right frames in fixed relationship to one another so that some observers see the three dimensional image at the wrong distance. Usually focusing of both eyepieces is accomplished simultaneously. Conventional stereoscopic viewers thus are not easily adaptable to a wide variety of people with different eye refractive power.
Still another disadvantage of conventional viewers is that efficient diffused illumination of the entire slide surfaces is difficult to achieve.
U.S. Pat. No. 1,808,176 dated June 2, 1931, describes a stereoscopic instrument manufactured for a time by the Ritter Dental Manufacturing Company, Inc., of Rochester, New York, as their "Intra-Oral Stereoscope" primarily for the study of stereoscopic dental X-ray films, although the patent description suggests its possible use for scenic stereo photographs also. When the Ritter instrument is focused for unaccommodated normal (emmetropic) vision on the film plane, one sees in the field of view part of the outer vertical edges of the film apertures of the instrument as well as part of the vertical edges of the retaining frame for the diffuser glass. In addition, the observer is quite aware of the effect of looking through a larger pipe into the field of view and notices that the front end of this "pipe" intrudes into the field of view and is in fairly sharp focus. The "pipe" is the stereoscopic image of the two lens tubes.
U.S. Pat. No. 3,376,381 dated Apr. 2, 1968, of H. L. Ratliff, Jr., provides for separate lateral motion of viewing lenses and kinescope screens, rather than keeping left lens and screen together and right lens and screen together.
British Pat. No. 599,029 dated Mar. 3, 1948, to Lane proposes a stereoscope hinged at the middle in front and provided with means for slightly altering the angle between the two lens-and-film units in order to accommodate a range of interpupillary distances. In the disclosure he suggests, "It can be arranged that the two units always remain parallel to each other and that they be moved nearer together or farther apart to suit eyes of differing separations . . . "
U.S. Pat. No. 2,484,591, granted Oct. 11, 1949 to S. I. Rochwite, and its commercial embodiment, the Stereo Realist system, has lenses of 35 mm focal length with parallel axes 70 mm apart in its camera, and its stereoscope uses lenses of about 42 mm focal length. Objects in the stereoscopic virtual image appear somewhat too small and somewhat too far away; that is, perspective is distorted. The system uses slides having rectangular apertures a fixed distance apart and mounts the transparencies a fixed distance apart in those apertures; this creates in the stereoscope the effect of a scene observed through a window some distance away. Separation of infinity points in Realist slide transparencies is 63.5 mm and the distance between corresponding aperture points is 62 mm. The stereoscope provides for a range of interpupillary distances from 59.5 to 66.5 mm.
The Realist type system precludes the achievement of orthostereoscopic results. Only an observer with interpupillary distance of 63.5 mm can align his visual axes and the stereoscope lens axes with the infinity points of the two transparencies and thus fuse at infinity the left and right images of a point that was at infinity in the original scene; even when he does that all nearer points will appear at incorrect distances. What happens when the observer's interpupillary distance is not 63.5 mm is even worse distortion of perspective. If one switches rapidly back and forth throughout the range of inter-lens-axis displacement while watching the stereoscopic virtual image, in order to obtain the optimum setting, a minor disadvantage of the fixed slide aperture separation and fixed transparency separation combined with variable interlens distance is resultant eye discomfort. Also, a very near object in a scene appears to be much closer than the "window" described earlier. For instance, branches of a tree in spring bloom can seem to protrude through the window toward the observer. This case might be acceptable if it were thought of as an artistic arrangement. But the same phenomenon in a closeup portrait of a person's head and neck might be called "the Sir Walter Raleigh effect"; the human experience tells the observer that a bodyless head floating in space on the near side of a window is strangely abnormal. Precise alteration of the transparency separation distance and of the aperture separation distance has been proposed by some to overcome such unreality. The problem of the "window", perspective distortion, and other disadvantages characterize the prior art.
Accordingly, it is an important object of this invention to provide an improved stereoscopic system that overcomes one or more of the disadvantages enumerated above.
It is a further object of the invention to provide an exceptionally realistic stereoscopic system consistent with the preceding object.
It is a further object of the invention to give the viewer an exceptionally wide angle of viewed field in true perspective, consistent with one or both of the preceding objects.
It is a further object of the invention to achieve one or more of the preceding objects while uniformly illuminating the slides in an efficient manner.
It is a further object of the invention to provide a stereoscopic viewer in accordance with one or more of the preceding objects that permits the user to individually adjust the focus for each eye.
It is a further object of the invention to achieve one or more of the preceding objects while allowing the user to adjust spacing between left and right viewing assemblies for optimum interpupillary distance.
It is still a further object of the invention to achieve one or more of the preceding objects while uniformly backlighting both left and right slides.
It is still a further object of the invention to achieve the preceding object with a relatively low cost bulb as the source of the light energy.